In cross-flow pressure driven membrane desalination systems, the feed stream is typically fed into a high pressure feed inlet of a pressure vessel that leads the pressurized saline solution on a flow path parallel to the membrane in the membrane element and a portion of the water passes through the membrane and exits by a low pressure connection from the same pressure vessel. The remaining saline solution with increased concentration of the retained solutes can exit from a second high pressure connection from the pressure vessel which is denoted as the concentrate port (see FIG. 1).
FIGS. 1A and 1B relate to prior art (EP1691915) The crossflow pressure vessel for pressure-driven membrane desalination showing the flow arrangement during forward flow (1A) and reverse flow (1B) in which the high pressure connections of stream Q1 and stream Q4 are switched between the left (L) side and the right (R) side of the pressure vessel, while continuing to remove the low pressure product stream (Q3) which passed through the membrane.
It has been shown (I&EC v 46, EP1691915) that in such pressure driven cross-flow desalination systems that by reversing the flow of a stream to be desalinated by switching the feed and concentrate connections to the high pressure ports of the pressure vessel, mineral scaling can be prevented by carrying this out before the induction time has been completed. This allows the use of no or little antiscalant to prevent the precipitation in the membrane elements. Furthermore it is a common practice that in high recovery pressure driven membrane desalination processes that one or more desalination membrane elements are placed in pressure vessels in stages such that more pressure vessels are in an upstream stage and that they communicate their concentrate stream to the feed ports of a fewer number of pressure vessels in the downstream stage. This is called a tapered flow arrangement of pressure vessels. This practice preserves a minimum cross-flow rate in the downstream pressure vessels that helps to prevent fouling by colloids, organics and biomaterials as well as to reduce the concentration of salts at the membrane surface. It has been revealed (European Patent Publication No. EP1893325, the entire disclosure of which is incorporated herein by reference) that by a particular use of valves arrangement, that the block of pressure vessels can be repositioned from downstream stage to an upstream stage and exchanged with block of similar number of pressure vessels from the upstream stage which are moved to the downstream stage while at the same time switching the concentrate and feed connections on the blocks of pressure vessels being repositioned. By so doing the end of the membrane element in the downstream stage that saw the highest concentration concentrate will now be exposed to the feed solution which has the lowest concentration and also the lowest concentration of sparingly soluble salts. This will allow a zeroing of the induction time as described in I&EC v46 and EP1691915, the entire disclosure of which is incorporated herein by reference.
In both flow reversal arrangements described in I&EC v46 and in the patent application concerning repositioning of the pressure vessels in the tapered flow arrangement EP1893325, all the membrane elements in the pressure vessels are periodically exposed to undersaturated solutions. On the other hand, the piping downstream of the switching valves of the second stage pressure vessels (e.g. VBf, VAf and VCb in FIG. 2, when these valves are downstream of the second stage pressure vessels) always see supersaturated solutions, that may be with little or no antiscalant. Therefore while scaling is prevented on the membranes in the pressure vessel, it may not be prevented in the piping downstream of these switching valves. Such scaling is particularly a possibility downstream of the pressure maintenance devices P/FV, when these are pressure reducing valves that could cause cavitation downstream where the pressure is released, as shown in stream 14 of FIG. 2. It is a common practice to place flow, conductivity and other composition sensors on the concentrate line downstream of the pressure maintenance device and scaling of these sensors could cause them to malfunction and misreport. This in turn could interfere with the appropriate control strategy of the plant. It is a purpose of the present invention to provide a practical solution to this problem that does not require the wasting of permeate and does not interrupt the smooth operation of the desalination system with flow reversal or repositioning of pressure vessels.
In EP1893325 a particular embodiment was described for repositioning pressure vessels by the use of three-way valves (see FIG. 2, wherein BW refers to Brackish water, P-I refers to Permeate (step I), P-II refers to Permeate (step II) and CT refers to Concentrate). While this method is effective, it can be problematic because of the brief time during which those valves must switch between one port and the other port of the three-way valve. Furthermore it does not easily allow for complete isolation of one block of pressure vessels which may be desirable for maintenance or diagnostic purposes while operating the rest of the desalination unit. Furthermore, while an auxiliary bypass valve (AV) was provided for taking part of the flow from the downstream block of elements when they were being switched to an upstream stage, the particular embodiment did not describe such a bypass valve for the upstream block of elements, so that during the brief time when a fewer number of blocks of pressure vessels was being operated in an upstream stage until a new block of pressure vessels could be repositioned into that block, it would see a much larger flow that could exceed the flow allowed. At the same time when a new block of pressure vessels was repositioned into the first stage, if all of its designed flow was immediately fed to this re-positioned block, it may be subject to water hammer or other mechanical stresses that could be harmful.